A measure of "electrical pressure" between two points. The voltage specification of an incandescent lamp is the electrical "pressure" required to drive it at its designed point. The "voltage" of a ballast (e.g. 120 V) refers to the line voltage coming from the power source and is connected to the ballast.

Volts

120

Length (in)

Also referred to as "Maximum Overall Length" (MOL). For light bulbs, it measures the length of a light bulb, from the top of the bulb to the bottom of the base or pins and is typically expressed in inches. For light fixtures and ballasts, it applies to the total horizontal length of the item.